The present invention generally relates to fiber gratings and, more particularly, to a tunable Bragg grating and laser.
It is known in the art of fiber optics that Bragg gratings embedded in the fiber may be used in compression to act as a tunable filter or tunable fiber laser, as is described in U.S. Pat. No. 5,469,520, entitled xe2x80x9cCompression Tuned Fiber Gratingxe2x80x9d to Morey, et al and U.S. Pat. No. 5,691,999, entitled xe2x80x9cCompression Tuned Fiber Laserxe2x80x9d to Ball et al, respectively, which are hereby incorporated herein by reference.
To avoid fiber buckling under compression, the technique described in the aforementioned U.S. Pat. Nos. 5,469,520 and 5,691,999 uses sliding ferrules around the fiber and grating and places the ferrules in a mechanical structure to guide, align and confine the ferrules and the fiber. However, it would be desirable to obtain a configuration that allows a fiber grating to be compressed without buckling and without sliding ferrules and without requiring such a mechanical structure.
Also, it is known to attach an optical fiber grating to within a glass tube to avoid buckling under compression for providing a wavelength-stable temperature compensated fiber Bragg grating, as is described in U.S. Pat. No. 5,042,898, entitled xe2x80x9cIncorporated Bragg Filter Temperature Compensated Optical Waveguide Devicexe2x80x9d, to Morey et al. However, such a technique exhibits creep between the fiber and the tube over time, or at high temperatures, or over large compression ranges.
The first aspect of the present invention is a tunable optical device, which comprises an optical waveguide having a longitudinal axis, a first mounting location and a second mounting location separated by a distance along the longitudinal axis, which transmits an optical signal, wherein the waveguide comprises a core and a cladding disposed outside the core, and wherein the cladding has an outside diameter and includes a first and a second variation region each having a modified outside diameter different from the outside diameter, wherein the first and second variation regions are respectively located at the first mounting location and the second mounting location, a Bragg grating imparted in the core of the waveguide between the first mounting location and the second mounting location, wherein the Bragg grating comprises a plurality of perturbations defined by a spacing along the longitudinal axis to partially reflect the transmitted optical signal at a reflection wavelength characteristic of the spacing of the Bragg grating, a first attachment mechanism disposed against at least one portion of the first variation region which prevents relative movement between the first variation region and the first attachment mechanism, a second attachment mechanism disposed against at least one portion of the second variation region which prevents relative movement between the second variation region and the second attached mechanism, a mounting device having a first end for fixedly mounting the first attachment mechanism and a second end which movably mounts to the second attachment mechanism and defines a separation length between the first and second attachment mechanisms along the longitudinal axis of the waveguide, and an adjustment mechanism, operatively connected to the second attachment mechanism, which adjusts the separation length, thereby causing a change in the distance between the first and second variation regions and the spacing of the Bragg grating to tune the reflection wavelength.
According to the present invention, the attachment mechanism comprises a first ferrule including a front portion having a profile substantially corresponding to the modified outside diameter of the first variation region of the cladding and a first butting mechanism butting the first ferrule against the waveguide to press the front portion of the first ferrule onto at least one portion of the first variation region at the first mounting location which limits relative movement between the first ferrule and the first variation region of the cladding, and the second attachment mechanism comprises a second ferrule including a front portion having a profile substantially corresponding to the modified outside diameter of the second variation region of the cladding and a second butting mechanism butting the second ferrule against the waveguide to press the front portion of the second ferrule onto at least one portion of the second variation region at the second mounting location which limits relative movement between the second ferrule and the second variation region of the cladding.
According to the present invention, the first butting mechanism provides a pressing force against the front portion of the first ferrule along a first direction substantially parallel to the longitudinal axis, and the second butting mechanism provides a pressing force against the front portion of the second ferrule along a second direction substantially opposite to the first direction.
According to the present invention, the waveguide further comprises a buffer layer over the cladding to protect the waveguide against the first and second attachment mechanisms and which enhances attachment of the first and second attachment mechanisms to the waveguide.
According to the present invention, the first and second ferrules comprise a plurality of pieces substantially surrounding the respective variation regions, which attach to the cladding.
According to the present invention, wherein a further waveguide segment including a cladding having a second outside diameter substantially equal to the modified outside diameter is spliced with the waveguide in order to provide each of the first and second variation regions.
According to the present invention, the modified outside diameter is provided by heating and stretching the waveguide to change the outside diameter of the cladding.
According to the present invention, the optical waveguide is an optical fiber.
According to the present invention, the adjustment mechanism can be a piezoelectric transducer, a stepping motor, a pneumatic force actuator, a solenoid or the like.
Furthermore, a section of the core between the variation regions, including the Bragg grating, is doped with a rare-earth dopant for forming a laser with the Bragg grating.
The second aspect of the present invention is a method of wavelength tuning an optical, wherein the optical device comprises an optical waveguide having a longitudinal axis to transmit an optical signal, wherein the waveguide has a first mounting location and a second mounting location separated by a distance along the longitudinal axis, and wherein the waveguide comprises a core and a cladding disposed outside the core; wherein the cladding has an outside diameter and includes a first and a second variation region each having a modified outside diameter different from the outside diameter, and wherein the first and second variation regions are respectively located at the first mounting location and the second mounting location; and a Bragg grating imparted in the core of the waveguide between the first mounting location and the second mounting location, wherein the Bragg grating comprises a plurality of perturbations defined by a spacing along the longitudinal axis to partially reflect the transmitted optical signal at a reflection wavelength characteristic of the spacing of the grating, said method comprising the steps of:
providing a first attachment mechanism disposed against at least one portion of the first variation region which prevents relative movement between the first variation region and the first attachment mechanism;
providing a second attachment mechanism disposed against at least one portion of the second variation region which prevents relative movement between the second variation region and the second attached mechanism;
providing a mounting device having a first end which fixedly mounts to the first attachment mechanism and a second end which movably mounts to the second attachment mechanism in order to define a separation length between the first and second attachment mechanisms along the longitudinal axis of the waveguide; and
providing an adjustment mechanism, operatively connected to the second mechanism, to adjust the separation length, thereby causing a change in the distance between the first and second variation regions and the spacing of the grating which tunes the reflection wavelength.
According to the present invention, the first attachment mechanism comprises a first ferrule including a front portion having a profile substantially corresponding to the modified outside diameter of the first variation region of the cladding and a first butting mechanism butting the first ferrule against the waveguide to press the front portion of the first ferrule onto at least one portion of the first variation region at the first mounting location in order to limit relative movement between the first ferrule and the first variation region of the cladding; and the second attachment mechanism comprises a second ferrule including a front portion having a profile substantially corresponding to the modified outside diameter of the second variation region of the cladding and a second butting mechanism butting the second ferrule against the waveguide to press the front portion of the second ferrule onto at least one portion of the second variation region at the second mounting location in order to limit relative movement between the second ferrule and the second variation region of the cladding.
According to the present invention, the method further comprises the step of providing a coating between the cladding and the first and second ferrules which helps the ferrules to conform with the outside diameter of the respective variation regions in order to reduce point contact stresses on the waveguide.
According to the present invention, the method further comprises the step of providing a buffer layer over the cladding which protects the waveguide against the first and second attachment mechanisms and enhances attachment of the first and second attachment mechanisms to the waveguide.
According to the present invention, the method further comprises the step of bonding the buffer layer to the first and second attachment mechanisms.
According to the present invention, the method further comprises the step of splicing a further waveguide segment including a cladding having a second outside diameter substantially equal to the modified outside diameter with the waveguide to form each of the first and second variation regions.
According to the present invention, the method further comprises the step of heating and stretching the waveguide to form the modified outside diameter of the first and second variation regions.
The present invention provides a significant improvement over the prior art by combining an optical fiber, having an expanded and/or recessed outer dimension variation region, with a structure, such as a ferrule or housing, having a size and shape such that the structure mechanically locks against at least a portion of the variation, thereby allowing the structure to attach to the fiber with minimal relative movement (or creep) in at least one predetermined direction between the fiber and the structure. The variation region and the structure may have various different shapes and sizes. However, while the geometry of the variation region is created from the optical fiber, low optical loss of the light being transmitted through the core of the fiber is maintained. There may also be a buffer layer between the cladding and the ferrule to protect the fiber and/or to help secure the structure to the fiber to minimize creep. Adhesives, such as solders, brazes, epoxies, etc., may also be used between the structure and the variation region.
The foregoing and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof.